In nuclear power generation facilities, automatic instruments have been installed in furnaces, the instruments in the furnaces have been accessed, and various maintenance treatments have been carried out, during periodic inspections. Laser processing methods and apparatuses, by which stress corrosion cracking (SCC) caused by tensile stresses remaining in welds can be effectively prevented from occurring, have been especially proposed as countermeasures against SCC. In particular, one of such laser processing methods is laser peening.
The laser peening is a metal surface treatment method carried out for improving the fatigue strength, abrasion resistance, corrosion resistance, and/or the like of a metal apparatus or the like. In the method, a surface of a metal is irradiated with pulse laser light, a shock wave generated in the irradiation propagates into the interior of the metal, and a more dynamic stress due to the shock wave than the yield stress of the metal results in plastic deformation. As a result, texture deformation or the like occurs in the interior of the metal, and a compressive residual stress is applied.
Metal surface treatment by such laser peening will be described specifically. First, a surface of a metal member as a member to be treated is irradiated with pulse laser light. In this case, for example, light is condensed on a spot having a diameter of around 1 mm through a condensing lens using a laser beam having a pulse width of around several nanoseconds (ns), and the metal member is irradiated with the light. The irradiation allows the surface of the metal member to absorb energy and to become plasma. When the surface of the metal member is covered with a liquid transparent to the wavelength of the laser beam, the liquid prevents the generated plasma from expanding to increase the internal pressure of the plasma. The pressure reaches, for example, around several gigapascals (GPa). The pressure shocks the member to then generate a strong shock wave. The shock wave propagates into the interior of the metal member to cause plastic deformation to apply a compressive residual stress.
Laser peening has the features of having a peening effect that is more insusceptible to material strength and the like than the peening effect of other peening such as shot peening or water jet peening, and that reaches the interior having a depth of around 1 mm from a surface of a member to be treated. The laser peening also has the features of hardly causing a reaction force to be generated during processing, and of allowing a processing apparatus to be easily downsized to result in excellent processability in a narrow portion.
However, it may be difficult to apply laser peening to the interior of a member to be treated having only very narrow space. For example, a nozzle is joined to the upper lid of a pressure vessel of a pressurized water reactor by welding, and a thermal sleeve is inserted into the interior of the nozzle. Even when laser peening has been intended to be applied to a spot in which such a nozzle is welded, only a gap of, for example, around 3 mm has existed between the inner surface of the nozzle and such a thermal sleeve, and therefore, it has been difficult to perform working in such a narrow portion using conventional laser peening.
For example, there is an idea that an optical fiber and/or the like are used in order to guide laser light to such a narrow portion; however, since laser light with which irradiation is performed from the leading edge of the optical fiber is subjected to irradiation in the axial direction of the optical fiber, change of a light path is required for irradiating a member to be treated, located in parallel to the insertion direction of the optical fiber, with laser light with which the irradiation is performed from the optical fiber. Thus, an apparatus in which a coreless fiber with a curved surface is connected to an end face of an optical fiber, and laser light with which irradiation is performed from the optical fiber is condensed in a direction perpendicular to the axial direction of the optical fiber has been proposed as a method for irradiating a surface of the member to be treated of such a narrow portion with laser light.
However, since the distance between the central axis of the optical fiber and a member to be processed is short as in the case of a narrow portion, an insufficient distance for condensing light can be made to result in a lower generated shock wave even when it is intended to condense light in a direction perpendicular to the axial direction of the optical fiber. In general, the energy of a shock wave generated by irradiation of laser light becomes maximum in a portion located immediately above a site irradiated with light. Therefore, irradiation with laser light in the perpendicular direction from the leading edge of the optical fiber allows the leading edge of the optical fiber to receive a shock wave having large energy, and therefore enables the leading edge of the optical fiber to be damaged.